The following press release has been issued by the British Embassy in Spain
“The UK Government wants to ratify the Withdrawal Agreement on our departure from the EU in the next few weeks. This would mean we would not participate in the European Parliament elections.
“However, there is a possibility that the UK will have to participate in the elections. In this scenario, if you wish to vote, you should ensure that you are registered either with your local electoral authorities here in Spain or in the UK.
“The European Parliament elections in Spain will be held on 26 May. If the UK remains in the EU at the time of the election, voting will take place in the UK on 23 May.
“The deadline for registering to vote in the local and European Parliament elections in Spain was 30 January 2019. If you are not already registered to vote in Spain, you could instead choose to vote in the UK, as long as you have been registered to vote in the UK within the last 15 years. The deadline for registering to vote in EP elections in the UK is midnight on Tuesday 7 May. The process can be done online and you can register to vote either by post or by proxy. It is important to be aware that you cannot vote in both countries.”
Find out more about voting in the European Parliament elections: https://europa.eu/youreurope/citizens/residence/elections-abroad/european-elections/index_en.htm
For details on how to register as an overseas voter with the UK Electoral Commission: https://www.yourvotematters.co.uk/can-i-vote/overseas